This year's early count of sea turtles nesting on Florida beaches is encouraging, though there are many unknowns in the numbers.

Welcome to the mysterious world of sea turtles, which spend much of their life far from Florida's beaches encountering fishing boats, oil spills, plastic trash and any number of other perils.

Nesting by loggerhead, green and leatherback turtles has been trending up in numbers for nearly the past five years along the state's coast. But there have been significant dips and climbs in their nest counts.

Green turtles, for example, have wowed researchers with their growing preference for Florida beaches. Yet while last year's nest count was surprisingly high, this year's is down, which experts expected.

Named for the color of their fatty tissue, green turtles go about nesting in a peculiar way. Their nest counts predictably are up and down every other year.

"To me, it's one of the great enigmas in sea-turtle biology," Llewellyn Ehrhart, a pioneering turtle researcher and professor emeritus at the University of Central Florida. "It seems it would have something to do with food availability and nutrition, but I don't know of anything in ocean biology that is so regularly scheduled high, low, high, low, year in and year out."

The turtle kings of Florida's coastline, loggerheads, which have huge heads relative to their 3-foot shells, deposited eggs in 46,885 nests this year.

That count was from the state's "index" locations at 26 beaches, where nest monitoring has been done by researchers using the same methods since 1989.

Index counts are done during a 109-day window, which means tallies are smaller than annual totals, but the index data are valued for detecting trends.

"Every year, it's always a little bit of a surprise for what we get," said Anne Meylan, a senior research scientist at the Fish and Wildlife Research Institute. "The biology of the animals is really complicated, and they live a long time, and they are affected by things far away."

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More sea turtles come to Florida, but reasons are mysterious